1. Field of the Invention
The invention relates to a heating apparatus.
2. Related Art Statement
In a system for producing semiconductors, a ceramic heater may be provided for heating a wafer so as to deposit a semiconductor thin film on the wafer from gaseous raw materials such as silane gas by means of thermal CVD or the like. So called two-zone heaters are known as such ceramic heater. Such a two-zone heater has a ceramic substrate and inner and outer resistance heat generators embedded within the substrate. Separate power supply terminals are connected to the respective heat generators so that electric power may be applied independently on the respective generators. The inner and outer heat generators may be thus independently controlled.
Examples of such two-zone heater include the following. Japanese patent publication 2001-102157A discloses a heater having a ceramic substrate and two layers of heating elements embedded in the substrate. The calorific values in the inner zone and outer zone of each heating element are controlled so that two-zone control system of controlling inner and outer zones may be realized.
Further, Japanese patent publication 2001-52843A discloses a heater having a disk-shaped ceramic substrate and a plurality of heating elements provided on the surface of the substrate. Each heating element has a shape of a ring. The heating elements are positioned concentrically and made of a sintered body of metal powder. Electric power may be supplied to the heating elements independently.
According to the disclosure in Japanese patent publication 2001-102157A, two layers of heating elements are embedded in the substrate. However, the thickness of the ceramic layers under and over each heating element in the substrate are different from each other. The heat capacities of the ceramic layers under and over each element are thus different from each other. Such a difference in heat capacities may induce excessive thermal conduction in the thickness direction of the substrate. The uniformity of temperature on a mounting face for a semiconductor may be thereby deteriorated so that the temperature distribution on the mounting face is out of specification. Moreover, the additional shaping steps needed to embed two layers of heating elements in a ceramic shaped body reduce the productivity of the heater. Furthermore, the two layers of heating elements demands a thicker ceramic substrate, so that the total heat capacity of the heater is increased which reduces its responsiveness during temperature ascending and descending stages.
According to the disclosure in Japanese patent publication 2001-52843, a connector for a terminal is provided for each heating element. The connector and terminal are exposed to atmosphere surrounding the heater. In particular, the atmosphere in a semiconductor producing system may contain various corrosive gases, or their plasmas, such as a processing gas or cleaning gas. If the connector and terminal are exposed to such a corrosive atmosphere, the heater may have a shorter operational life. Moreover, at a back face of the ceramic substrate, it is necessary to provide a wiring construction to be connected with each of the heating elements. Such a wiring system may be complex and complicated. Furthermore, a complicated structure is also needed for fixing the heater to a semiconductor producing chamber, because of the complicated wiring construction at the back face of the heater.